As conventionally-known vertical semiconductor element that is made from an SiC substrate and has a trench gate structure, JP-A-2009-188221 and JP-A-2009-289987 disclose vertical metal-oxide semiconductor field-effect transistors (MOSFET). In the vertical MOSFETs disclosed in JP-A-2009-188221 and JP-A-2009-289987, in order to reduce on-state resistance in the vertical MOSFETs, a channel mobility is required to be increased. The channel mobility of SiC is dependent on a plane direction. Therefore, in the vertical MOSFET having the trench gate structure, a plane direction of a main surface of the SiC substrate and a plane direction of a sidewall of the trench are set in such a manner that the sidewall of the trench is set along a crystal plane in which the channel mobility is high. Specifically, a Si-face, which is a (0001) Si plane, or a C-face, which is a (000-1) C plane, is used as the main surface of the SiC substrate and an anisotropic etching is performed on the Si-face or the C-face to form the sidewall of the trench parallel to a (11-20) plane.
However, even if the Si-face or the C-face is used as the main surface of the SiC substrate and the anisotropic etching is performed to form the sidewall of the trench parallel to the (11-20) plane, the sidewall of the trench is formed to a tapered shape, which has a larger width at an entrance portion than at a bottom. Therefore, it is difficult to form the sidewall of the trench exactly parallel to the (11-20) plane. When the sidewall of the trench is not parallel to the (11-20) plane, the channel mobility will be decreased.